Active matrix color liquid crystal displays (LCDs) generally include five different components: a white light source, a first polarizing film that is mounted on one side of a display panel including an array of pixels defined by arrays of pixel electrodes and thin-film pixel transistors (TFTs), a color filter plate containing at least three primary colors arranged into pixels covered with a transparent common electrode and finally a second polarizing film. A liquid crystal material is disposed between the pixel and common electrodes. This material will rotate the polarization of light when an electric field is applied across it. Thus, when a particular pixel of the display is turned on, the liquid crystal material rotates polarized light transmitted through the material so that it will pass through the second polarizing filter.
For pixel TFTs, various types of silicon have been used, including amorphous silicon, polycrystalline silicon, and single crystalline silicon. Amorphous silicon TFTs generally have poor transistor performance because of very low electron mobility of amorphous silicon. As a result, the amorphous silicon TFTs can only deliver low current density, limiting their use to large-size LCD panels that can accommodate large-size TFTs. Polycrystalline silicon TFTs can in part address the problem associated with amorphous silicon TFTs because the electron mobility of polycrystalline silicon is much higher, but these materials still have a lower electron mobility than single crystalline silicon as they have crystalline defects (such as grain boundary) which can impact TFT performance. Single crystal TFTs have the highest performance, and they can be used for displays with very high pixel density, for example, higher than 100 pixels per inch. In addition, single crystal TFTs have high frequency response, allowing the control electronic circuits to be monolithically integrated within the display panel.
Generally, conventional single crystal TFTs employed in LCDs in the art have a relatively thick silicon layer, for example, having a thickness of about 300 nm or greater. With such thickness, the pixel TFTs may have relatively high photosensitivity (e.g., backlighting and/or ambient light photosensitivity), which can result in current leakage of pixel TFTs. Such current leakage of pixel TFTs, in turn, can cause a relatively low display contrast ratio and/or high image retention.
Thus, a need exists for a method of forming a high quality TFT at each pixel of an LCD that can address problems associated with conventional TFTs, especially forming TFTs that can operate at high speeds, that have reduced photosensitivity, and that can be fabricated easily and at low cost.